1. Field of the Invention
This invention generally relates to optical telecommunications and, more particularly, it relates to a method of identifying a particular optical path out of a plurality of optical paths in the form of optical fibers housed in a duct or pipe or laid in a telecommunications station for optically transmitting low frequency signals and/or other signals in an optical telecommunications system.
2. Prior Art
In recent year, the demand for optical fibers is remarkably expanding and optical LAN systems are widely used for so-called intelligent buildings. Such a system normally comprises a large number of optical paths housed in ducts and pipes and much more optical paths are expected to be installed for subscribers' networks in the near future.
Once optical paths are established in a duct or pipe, a problem may arise in identifying a particular optical path when it has to be singled out for removing or branching because all the optical paths installed there resemble each other and there is no way of identifying a particular one by appearances.
The problem of singling out a particular optical path becomes very serious when hundreds of optical fiber cores are densely arranged within a single optical cable. If a live optical path is mistakenly disconnected in an operation of connecting or removing optical fiber cores, the result can be disastrous in the modern society where people heavily rely on transinformation for sustaining their lives. Therefore, optical paths in modern optical telecommunications systems have to be identified accurately and rigorously by any means.
There has been proposed a method of identifying a particular optical path, utilizing fluctuations in the polarized light passing through the single mode optical fiber. With this method, an external signal typically in the form of an ultrasonic wave is applied to an optical path in an optical cable or the like from outside by utilizing the Faraday effect or some other acousto-optical effect to transmit the oscillation of the externally applied signal to the single mode optical fiber in the cable so that the optical signal being transmitted through the optical path may become fluctuated by the externally applied signal. Then, the particular single mode optical fiber can be identified by observing the fluctuations in level of the optical signal running through the optical fiber.
This will be described further by referring to FIGS. 5 and 6 of the accompanying drawings. Referring firstly to FIG. 6, polarized light from light source (LD) 23 is made to pass through optical path 1 of a single mode optical fiber and, while the polarized light is passing through the optical path, an external signal in the form of an ultrasonic wave is externally applied to the optical path 1 by means of an external optical modulator 21 that utilizes an acousto-optical effect so that the optical signal being transmitted through the single mode optical fiber may be modulated for the plane of polarization. Then, the light passing through light detector 29 arranged upstream to light receiving device (O/E) 31 is checked for fluctuations in the level of the signal arriving through the optical path to see if it is the right optical path to be singled out.
The external optical modulator 21 typically comprises a piezo-electric device for converting an electric signal into an acoustic signal. In actual applications, three regularly and angularly spaced external optical modulators 21 may be arranged around optical path 1 so that three acoustic signals may be applied to it from three different directions, although, alternatively, more than three external optical modulators may be used if they are angularly spaced at regular intervals around the optical path.
It may be needless to explain the significance of arranging a plurality of external optical modulators 21 around the outer periphery of a duct or cable containing a large number of single mode optical fibers in the operation of identifying a particular single mode optical fiber out of them when their relative positions are not known at a particular location along the duct or cable. With such an arrangement, electric signals may be applied to the respective external optical modulators 21 simultaneously or sequentially.
With the arrangement of optical signal transmission of FIG. 6, the state of polarization of the optical signal received by the light receiving device can be affected by external turbulences and/or the residual stress in the single mode optical fiber to become unstable and show significant fluctuations. In an attempt to get rid of this problem, there has been developed a technique of polarization diversity reception, with which the optical paths in a duct or cable are divided into groups facing different directions so that a group that is currently most stable may be used for optical telecommunications. Another known technique developed to counter the problem is the use of a polarization scrambler that can randomly modulate the state of polarization to artificially produce a best state and a worst state of polarization.
While any particular optical path 1 has to be accurately identified by any means in a modern optical telecommunications network as emphasized earlier, the known modulation techniques are disadvantageous in that they also can be vitally and adversely affected by external turbulences and other causes of troubles because they are designed to modulate the optical signal in an optical path for the plane of polarization by applying an external signal to it. In other words, where external turbulences exist, the signal modulated by an external signal for the plane of polarization cannot be identified with a reliability of 100%. Therefore, with any of the above described known techniques of screening a number of optical paths transmitting respective optical signals simply on the basis of "presence" or "absence" of a modulated signal, it is difficult to accurately identify a particular optical path.
A known proposed solution for the problem of accurately identifying a particular optical path transmitting an optical signal that has been modulated for the plane of polarization consists in lowering the threshold for discriminating optical signals. However, with a lowered threshold level, noises can be received as signal components which makes it difficult to single out a particular optical path transmitting a modulated optical signal. Therefore, this idea is not feasible for solving the problem of accurately identifying a particular optical path. Additionally, when a large noise is produced abruptly, it can be mistaken for an optical signal and erroneously received by the receiver to give rise to another problem in identifying a particular optical path out of large number of optical path.
In view of these circumstances, it is therefore the object of the present invention to provide a method for optical telecommunications with which an optical signal modulated for the plane of polarization can be received for certainty without lowering the threshold level and therefore a particular optical fiber (to which an external signal has been applied) can be accurately identified out of a plurality of optical fibers.